This invention relates to a non-volatile, high-density, solid magnetic memory, and more particularly to a magnetic memory using superconductor.
Solid magnetic memories can be classified broadly into a random access type and a serial access type. A core memory is a typical example of the former and a bubble memory, a typical example of the latter. To accomplish a high-density memory, the random access type involves the drawback that there is a limit to miniaturization of a cell size because a sensor is necessary for each bit. Though the serial access type can accomplish a high storage density relatively easily, it is not free from the critical problem in that the access time increases with an increasing storage density. Furthermore, those memory devices such as a bubble memory which require movement of bubbles as an information carrier have the drawback that stability of information is deteriorated with the movement. Under these circumstances it is desirable to develop a sensor for the random access memory and to accomplish its high storage density as a non-volatile solid magnetic memory.
The drawback with the core memory as the typical example of the random access memory using a magnetic material has lain in that the cell size is large and high storage density cannot be accomplished easily.
To accomplish a high storage density, a magnetic memory using a magnetic thin film has been proposed. The magnetic thin film is, for example, a 19% Fe-81% Ni alloy as a soft magnetic material having a magneto-striction constant .lambda.=0 and is vacuum deposited in a disk-like form at the point of intersections of conductor wires disposed in both longitudinal and transverse directions. However, this memory involves the problems that stability of stored information is not high because magnetization of the disc-like magnetic film returns gradually to original directions due to the interaction with the adjacent cells and when the magnetic pattern is minituarized, the detection output becomes small and information read-out becomes difficult.
In order to solve these problems, the inventor of the present invention proposed already (not published) a high-density solid magnetic memory in my copending U.S. patent application Ser. No. 07/302,673, now an allowed application. In this magnetic memory, a ring structure made of a superconductor material is introduced, stability of stored information is improved by utilizing a diamagnetic current occurring in this superconductor ring and read-out of stored information is made easier by utilizing the transition between superconduction state and normal state of the superconductor ring. Its basic structure is as follows. In a magnetic memory including a ferromagnetic film having uniaxial magnetic anisotropy within the plane of a film on a substrate and first and second superconductor wires disposed in such a manner as to cross each other to sandwich the magnetic film at a cross-over area of the two wires, the present invention disposes a third superconductor wire in such a manner as to be overlaid on the first superconductor wire and to encompass the magnetic film pattern and the second superconductor wire and disposes further another conductor wire or superconductor wire in such a manner as to be overlaid on the superconductor wires at positions where the first and third superconductor wires cross the second superconductor wire.
In this memory, the ring formed by the first and third superconductor wires must once be switched to the normal state at the end of the magnetization reversal process (the information write process for each cell) in order to reverse the magnetization of the magnetic pattern inside the ring formed by the first and third superconductor wires. The transition between the superconductor state and the normal state is generally attained by applying a current above a critical current to the superconductors forming the ring, but when this method is employed, the critical currents for the first and third superconductors must be reduced as much as possible in order to keep power consumption of the memory at a low level. If the critical current is reduced, on the other hand, the force for confirming the magnetic flux inside the ring becomes weak and there takes place the limit to miniaturization of the cell size that is the greatest target. Since the difference is provided in the critical currents of the first and third superconductors in order to read out information, the critical currents of the superconductors must be controlled accurately. However, the superconductors that can be utilized are generally two kinds, and the phenomenon that the magnetic flux leaving the magnetic pattern is trapped by the superconductor ring makes the problem all the more complicated. Thus, the superconductor ring involves the serious drawback irrespective of its merit.